Eating a quarter of a grapefruit every day can increase the risk of developing breast cancer in post-menopausal women by almost a third...

Eating a quarter of a grapefruit every day can increase the risk of developing breast cancer in post-menopausal women by almost a third, The Daily Telegraph reported. The newspaper went on to explain that "grapefruit boosts blood levels of the hormone oestrogen, which is linked with the risk of breast cancer".

The Telegraph reported the authors as saying: "[As] it is well established that oestrogen is associated with breast cancer risk, it is plausible that regular intake of grapefruit would increase a woman’s risk of breast cancer." According to the paper, the authors concluded that eating grapefruit every day was "significantly associated" with an increased breast cancer risk.

The rates of cancer in women who ate grapefruit were compared with those who did not and the authors have selectively reported an adjusted result in the women who ate the most grapefruit. From this data it is not possible to verify a causal link between grapefruit intake and cancer incidence.

Where did the story come from?

The study was conducted by Dr Kristine Monroe and colleagues in the Department of Preventive Medicine at the University of Southern California. It was supported by a grant from the US National Cancer Institute and was published in the peer-reviewed journal British Journal of Cancer.

What kind of scientific study was this?

This prospective observational study looked at a subset of postmenopausal women from a larger study. The main study (the Multiethnic Cohort Study) collected data on 46,000 women between 1993 and 1996 and then those who developed breast cancer – 1,657 women – were tracked.

The information on diet, including grapefruit intake, was collected in a questionnaire that was filled in by the women at the start of the study. This data was analysed for the women who developed breast cancer through the study.

What were the results of the study?

The researchers report that high grapefruit intake (defined as 60g or about a quarter of a grapefruit, or more per day) "increased the risk of breast cancer" by about 30% compared to not having any grapefruit. This estimate was based on an adjustment for 16 other factors, for example family history and smoking.

What interpretations did the researchers draw from these results?

The researchers interpreted these results to mean that grapefruit intake may increase the risk of breast cancer among postmenopausal women.

There are potentially some problems with this study that should make women cautious about concluding that eating grapefruit is dangerous. The results cannot be interpreted in the way that media and researchers have done, without further confirmation, because:

The reporting of the methods the researchers used to examine the link between breast cancer and grapefruit intake is ambiguous as and it is not clear how the risks have been calculated or adjusted;

The study was not able to be fully evaluated, in particular the influence of age on breast cancer risk;

The intake of grapefruit juice was not taken into account;

The quality of the larger prospective study – the Multiethnic Cohort study – from which this subset of women was drawn will have an effect on the quality of this secondary study.

Sir Muir Gray adds...

The late Sir Richard Doll and Sir Richard Peto, the UK’s leading cancer epidemiologists, estimated that one third of cancers are caused by dietary factors. The results of this study need to be integrated with the results of other studies regarding the factors associated with a higher rate of breast cancer in a systematic review of the scientific literature.